In an X-ray computer tomography apparatus using an X-ray cone beam, an X-ray generator and an X-ray detector interposing an object is rotated around the object relative to the object. During the rotation, the X-ray generator generates an X-ray cone beam having a cone or pyramid shape to irradiate the object in various directions, and the X-ray detector having a two-dimensional detection plane measures an intensity distribution of X-rays transmitting the object or a projection of the object. The acquired projection data is back-projected to reconstruct the distribution of X-ray absorption coefficients and to create a two-dimensional tomographic view or a three-dimensional stereoscopic view.
An X-ray detector for a wider imaging area is generally more expensive. Therefore, it is proposed to image a wider area with a less-expensive X-ray detector having a smaller detection area. In a normal X-ray cone beam CT imaging the entire imaging area is always included in an X-ray irradiation field, and the projection in the entire imaging area is detected. The image reconstruction can be carried out by using projection data obtained during a rotation of 180 degrees. On the other hand, X-ray cone-beam CT imaging apparatuses described in JP-A 2002-204796 and 2005-6772 shift detection center in the X-ray detector in a direction perpendicular to a line connecting the X-ray detector and an axis of the object (or rotation center) for CT imaging. When the shift becomes larger, the X-ray detector detects, in each instant, not the entire area to be imaged, but a part thereof. However, by using projection data obtained during a rotation of 360 degrees, an image of an area wider by the offset of the detection center can be reconstructed. If the position of the X-ray detector is shifted so that the line connecting the X-ray detector and the rotation center reaches to an edge of the detection plane, the X-ray detector detects, in each instant, the X-rays transmitting a half of the area of interest to be imaged, while the width of the imaged area during the rotation of 360 degrees become twice in contrast to the normal CT imaging.
In an X-ray CT imaging apparatus described in JP-A 2007-29168, a distance between an X-ray generator (and/or an X-ray detector) and the revolution center is changed relatively in order to change the magnification factor. Because the revolution center of the X-ray generator and the X-ray detector is different from the center of a region of interest in an object, the positions of the X-ray generator and the X-ray detector are controlled so that the relative position relationship between the X-ray generator, the object and the X-ray detector is kept constant. In a X-ray cone-beam CT imaging apparatus described in JP-A 2007-143948, one rotation driver can be used for CT and panorama imaging. However, CT imaging with an offset scan cannot be performed. In the apparatus, the rotary arm is set below a patient. Further, In an X-ray cone-beam CT imaging apparatus described in JP-A H09-327453, an X-ray tube and an X-ray image intensifier (X-ray detector) are mounted opposing to each other in a gantry, and they can be moved in the gantry along an arc. Thus, the field of view of a tomographic section in an X-ray CT image can be enlarged. However, the apparatus cannot perform panorama imaging.
As explained above, the size of a reconstructed image can be changed largely by shifting the position of the X-ray generator relatively to that for the normal CT imaging. However, it is desirable to image a still larger region of interest of an object.